There is disclosed in the related art an orthogonal frequency-division multiplexing (OFDM) demodulation device having a Viterbi decoder configured to decode a convolutional code, a fading determination unit configured to determine a fading level of a primary demodulated signal, and a soft-decision bit selector configured to select from the demodulated signal soft-decision bits to be input into the Viterbi decoder. The soft-decision bit selector selects part of the demodulated signal as soft-decision bits based on the fading level determined by the fading determination unit. Further, the fading determination unit computes the maximum amplitude difference between the highest amplitude value and the lowest amplitude value of each of subcarriers in the propagation path estimating preamble to determine a fading level based on the computed maximum amplitude difference (e.g., Patent Document 1).
In addition, there is disclosed in the related art a receiver having a demodulator configured to soft-decision demodulate a received signal, a mode detector configured to detect the mode in signal amplitudes based on a frequency distribution of the soft-decision signal amplitudes in a decoding unit-interval of an output signal of the demodulator, a normalization unit configured to normalize soft-decision signal series based on the detected mode, and a decoder configured to decode received data based on the normalized soft-decision signal series (e.g., Patent Document 2).
In the related art OFDM demodulation device, the fading level is determined based on the maximum amplitude or the variance between the highest value and the lowest value of each of the subcarriers to optimize a soft-decision bit scale of likelihood information. Hence, in the related art technique, the appropriate likelihood may be obtained under a frequency selective fading environment where a level of receiving a specific frequency is lowered.
However, under an environment where spurious noise is generated against a specific frequency, the effect of the spurious noise is mixed with the effect of the frequency selective fading. Hence, the appropriate mode (likelihood) will not be obtained by the normalization of the soft-decision signal series alone.